The present invention relates to a silicone rubber composition and more particularly the present invention relates to a one component self-extinguishing room temperature vulcanizable silicone rubber composition.
At the present time there has been a large emphasis made in the industry to obtain flame inhibitive compositions for articles that are utilized by consumers or for industrial use such as, in the fabrication of electrical components. It has been especially desirable in the construction of electrical components to utilize sealants which would be self-extinguishing and more specifically to utilize silicone sealants which are self-extinguishing. Generally, silicone compositions that cure to silicone elastomers, while being slower to catch on fire than other construction sealants, do nevertheless, when the temperature of burning exceeds a certain temperature limit burn or support combustion. Accordingly, it has been found desirable to formulate silicone materials that were self-extinguishing or would support combustion only for a limited period of time, unless, of course, the temperature went to exceptionally high limits.
One effort in this direction was as set forth in the disclosure of Noble and Brower, U.S. Pat. No. 3,514,424, that comprises the incorporation of platinum compounds in heat vulcanizable silicone rubber compositions which improved the flame inhibitive properties of heat vulcanizable silicone rubber compositions. However, such platinum additives which were effective in improving the flame retardancy of heat vulcanizable rubber compositions were not that altogether effective in improving the flame retardancy of room temperature silicone rubber compositions. Specifically, heat vulcanizable silicone rubber compositions comprise a polysiloxane gum, a filler as well as the flame retardant additive and a peroxide catalyst where the composition is heated at elevated temperatures above 100.degree. C. to produce a silicone elastomer.
Room temperature vulcanizable silicone rubber compositions are on the other hand broken down into two-component room temperature vulcanizable silicone rubber compositions and one-component room temperature vulcanizable silicone rubber compositions. The two-component system is distinguished from the one-component in that other than the difference in ingredients, the two-component system cures upon mixing the two-components in situ at room temperature either in the presence or absence of atmospheric moisture to produce a silicone elastomer that is fully cured within 24 hours. The one-component room temperature vulcanizable silicone rubber composition comprises a silanol polymer, a filler and a cross-linking agent selected from alkoxy functional silanes, acetoxy functional silanes, ketoximino functional silanes, amide functional silanes, amino functional silanes and other types of functional silanes as is well known in the one-component room temperature vulcanizable silicone rubber compositions prior art. One-component room temperature vulcanizable silicone rubber compositions are packaged in anhydrous state and sealed in a package such as, a caulking tube in the absence of moisture. When such composition is exposed to atmospheric moisture the cross-linking agents hydrolyze and this results in the formation of a silicone elastomer whereupon final cure does not take place for 24 to 72 hours. Such one-component room temperature vulcanizable silicone rubber compositions are highly desired as sealants, adhesives and encapsulating agents. Accordingly, the use of the Noble and Brower platinum additive to such one-component room temperature vulcanizable silicone rubber compositions did not markedly improve the flame retardant properties of such compositions. Another approach was to incorporate chlorinated hydrocarbon compounds in such one-component room temperature vulcanizable silicone rubber composition which chlorinated hydrocarbon compounds improved the flame inhibitiveness of such cured silicone elastomers. However, undesirably when such compositions were burned at excessively high temperatures there was given off in some cases toxic by-products.
Accordingly, it was highly desirable to obtain a non-toxic self-extinguishing one-component room temperature vulcanizable silicone rubber composition. One approach to this problem is as disclosed in the Patent of Shingledecker, U.S. Pat. No. 3,734,881. In accordance with the disclosure of this patent a non-toxic self-extinguishing one-component room temperature vulcanizable silicone rubber composition is obtained by having in addition to the silanol base polymer and the necessary fillers, a platinum compound, carbon black, a metal oxide filler such as, zinc oxide, optionally some phenyl in the base polymer and where the cross-linking agent is a vinyl-containing acetoxy functional silane or a vinyl-containing ketoxime functional silane. While such composition is a self-extinguishing one-component room temperature vulcanizable silicone rubber composition and gives off a minimum of toxic by-products upon burning, nevertheless, it does give off corrosive by-products when the cross-linking agent hydrolyzes. Accordingly, it was highly desirable to obtain for the encapsulation of electrical components a non-toxic, non-corrosive, self-extinguishing room temperature vulcanizable silicone rubber composition. However, in Column 4 of the Shingledecker U.S. Pat. No. 3,734,881, beginning with line 64, the patentee discloses that other useful moisture curable room temperature vulcanizable silicone elastomers are not made self-extinguishing by the combination of carbon black and platinum. Accordingly, it was highly unexpected to discover a self-extinguishing one-component room temperature vulcanizable silicone rubber composition which gives off a minimum of toxic by-products while burning and also does not evolve corrosive by-products during cure.
It is one object of the present invention to provide for a one-component room temperature vulcanizable silicone rubber composition which is self-extinguishing. It is another object of the present invention to provide a one-component room temperature vulcanizable silicone rubber composition which gives off a minimum of toxic by-products during burning and does not give off any corrosive by-products during its cure. It is an additional object of the present invention to provide for a process for producing a one-component room temperature vulcanizable silicone rubber composition which is self-extinguishing, gives off a minimum of toxic by-products upon burning and does not give off any corrosive by-products during cure. It is yet an additional object of the present invention to provide for a one-component room temperature vulcanizable silicone rubber composition which has an optimum combination of self-extinguishing and flame inhibitive properties. These and other objects of the present invention are accomplished by means of the disclosure set forth hereinbelow.